Image forming apparatus and image forming method

ABSTRACT

An image forming apparatus including (a) a γ conversion unit having a γ conversion table γα i  of a first one Vcontα of a designation development contrast in a case that a development contrast correction amount ΔVcont 1  is a first value, and a γ conversion table γβ i , different from the γ conversion table γα i , of a second one Vcontβ, different from the first one Vcontα, of the designation development contrast in a case that the development contrast correction amount ΔVcont 1  is a second value different from the first value, and (b) an arithmetic operation unit for calculating a γ conversion table γNow i  of a third one VcontNow of the designation development contrast in a case that the development contrast correction amount ΔVcont 1  is a third value, based on a formula: γNow i =γα i +(γβ i −γα i )×(Vcontα−VcontNow/(Vcontα−Vcontβ), wherein the subscript i is a value of an output density signal in the γ conversion table. The γ conversion unit conducts the γ conversion of the third one VcontNow of the designation development contrast, based on the γ conversion table γNow I  calculated by the arithmetic operation unit.

This is a divisional of U.S. patent application Ser. No. 11/749,884,filed May 17, 2007, which is a divisional of U.S. patent applicationSer. No. 11/080,490, filed Mar. 16, 2005, now U.S. Pat. No. 7,259,885,which is a divisional of U.S. patent application Ser. No. 09/797,991,filed on Mar. 5, 2001, now U.S. Pat. No. 6,987,576.

This application claims the right of priority under 35 U.S.C. § 119based on Japanese Patent Application No. 2000-060284, filed on Mar. 6,2000, and Japanese Patent Application No. 2000-223726, filed Jul. 25,2000, which are hereby incorporated by reference herein in theirentirety as if fully set forth herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image forming apparatus, such as acopying apparatus or a printer, and, more particularly, to gamma (γ)conversion for use therein.

2. Related Background Art

For adjusting image density in an image forming apparatus, there areknown digital image processing method and analog adjustment methods. Thedigital image processing methods include an adjustment withluminance-density conversion means for converting a received originalluminance signal into an original density signal, an adjustment withoutput density conversion means for converting an original densitysignal into an output density signal, and an adjustment constituted byan inverse function of printer characteristics, representing an outputdensity signal corresponding to an output signal. In general, theadjustment by the gamma conversion means is easiest for adjusting theimage density, and is more direct and accurate in determining theprinter characteristics.

For analog adjustment, it is common to adjust the potential of thelatent image or the development potential. There are other adjustableparameters, such as toner amount of the development means, developmentfrequency, various adjusting gaps in the development means and a gapbetween the development means and the electrophotographic photosensitivemember, but the latent image potential and the development potentialhave conventionally been employed in consideration of each of theadjustments. Particularly, the most common parameter is the developmentcontrast, which is the difference between the latent image potentialinducing the toner adhesion on the electrophotographic photosensitivemember and the development potential of the development means.

The digital image adjustment represented by the adjustment of thecontent of conversion of the gamma conversion means is capable ofrelatively adjusting the density in the halftone area according to thedesired amount, but cannot adjust the density of an area in which thetoner is deposited with the maximum amount (hereinafter represented asDmax) or a fog level in an area where the toner is not deposited.

On the other hand, the analog image adjustment represented by theadjustment of the development contrast can adjust, not only the densityof the halftone area, but also, the maximum amount Dmax in mutuallinkage. The fog level in the area without the toner adhesion scarcelychanges unless the back contrast, relating to the difference between thedevelopment potential and the potential in the area without the toneradhesion, is altered.

Also, the digital image adjustment and the analog image adjustment, asdescribed in the foregoing, may conventionally be designatedindividually, but there has not existed an algorithm or a sequencecapable of automatically linking the two in the image forming apparatus.

It is to be noted that there may result a situation where thedevelopment contrast has to be altered, not for the purpose of imageadjustment, but, for example, for reducing the toner consumption amountor for avoiding leakage of the charging means, for providing thephotosensitive member with a potential. As explained in the foregoing, achange in the development contrast alone results in a change in thedensity of the halftone area and Dmax in linkage. The resulting image,therefore, provides an impression of significantly deteriorated imagequality.

SUMMARY OF THE INVENTION

In consideration of the foregoing, the object of the present inventionis to provide an image forming apparatus and an image forming methodcapable of maintaining a halftone density by gamma conversion, therebyoutputting an image with little deterioration in the image quality, evenwhen the image contrast has to be altered, not for the purpose of imageadjustment.

The present inventors have investigated means for avoiding theimpression of deteriorated image quality in the image, even in a case ofaltering the development contrast for the purpose of reducing the tonerconsumption amount or for avoiding leakage of the charging means. As thebasis of the present invention, the present inventors have found thefollowing two experimental rules.

A first rule is that the impression of deterioration of an image isprincipally caused by the density of the halftone area, and is,therefore, not much dependent on Dmax, and that an impressionsatisfactory to most users can be given, even if Dmax is somewhatlowered, as long as the density can be maintained in the halftone area.For example, in the ordinary visual observation of the image, the imagequality does not appear to be deteriorated if a reflective density atleast equal to 1.1 is obtained.

A second rule is that the toner consumption amount is reduced by a lowerDmax, even if the density of the halftone area is made somewhat higher.This phenomenon is based on a fact that the deposited amount of toner islarger in the Dmax area than in the halftone area.

In consideration of these two experimental rules, an optimum image canbe provided if the halftone density can be maintained by an adjustmentof the content of conversion in the gamma conversion means when thedevelopment contrast is altered, since such a change in the developmentcontrast lowers the Dmax to reduce the toner consumption amount and toprevent the leakage in the charging means, while the image does not givea deteriorated impression, because of the maintained halftone density.More specifically, if the development contrast becomes deficient, thecontent of the gamma conversion means is adjusted so as to provide ahalftone density higher than that in a normal case.

In the present invention, therefore, there is adopted a configuration ofreading the development contrast in the printer portion, and the gammaconversion means provides a result of conversion corresponding to such adevelopment contrast.

Also, except for the aforementioned adjustment of the developmentcontrast not intended for image adjustment, such as for reducing thetoner consumption amount or for avoiding a leak in the charging means,the convent of conversion of the gamma conversion means is not adjustedin a case of the adjustment of the development contrast, for the purposeof image adjustment. This is because, in a case of reducing the halftonedensity by reducing the development contrast, for example, in the imagedensity adjustment for the purpose of general image adjustment or copingwith the environment fluctuation of the development means, if thecontent of conversion of the gamma conversion means is adjusted, suchimage processing results in an increase in the halftone density, thusattenuating the effect of the present invention.

The aforementioned object of the present invention can be attained,according to the present invention, by an image forming apparatusprovided with:

luminance-density conversion means for converting an original luminancesignal into an original density signal;

An image forming apparatus, which visualizes an image, according to alatent image potential, on an image bearing body, which includes acharging unit and an exposing unit, in which a development potential isapplied to a developing unit, and a development contrast is a differencebetween the latent image potential and the development potential,wherein a designation development contrast is defined as a sum of abasic development contrast Vcont, said image forming apparatuscomprising:

(a) a γ conversion unit having a γ conversion table γα_(i) of a firstone Vcont α of the designation development contrast in a case that thedevelopment contrast correction amount ΔVcont1 is a first value, and a γconversion table γβ_(i), different from the γ conversion table γα_(i),of a second one Vcontβ, different from the first one Vcontα, of thedesignation development contrast in a case that the development contrastcorrection amount ΔVcont1 is a second value different from the firstvalue; and

(b) an arithmetic operation unit for calculating a γ conversion tableγNow_(i) of a third one Vcont Now of the designation developmentcontrast in a case that the development contrast correction amountΔVcont1 is a third value, based on a formula:γNow_(i)=γα_(i)+(γβ_(i)−γα_(i))×(Vcontα−VcontNow/(Vcontα−Vcontβ)wherein the subscript i is a value of an output density signal in the γconversion table, and wherein

the γ conversion unit conducts the γ conversion of the third oneVcontNow of the designation development contrast, based on the γconversion table γNow_(I) calculated by the arithmetic operation unit.an]] output density conversion means for converting the obtainedoriginal density signal into an output density signal;

gamma conversion means for converting a signal into an output signalcorresponding to the output density signal;

charging means for providing an electrophotographic photosensitivemember with a uniform charge;

digital exposure means for exposing the electrophotographicphotosensitive member after the charging to the output signal after thegamma conversion, thereby forming a latent image; and

development means for developing the latent image with toner to obtain avisible image,

in which the image is rendered visible by forming a latent imagepotential, with the surface potential of the electrophotographicphotosensitive member formed by the charging means and the exposuremeans, and a development potential applied to the development means, theapparatus comprising:

environment detection means for detecting at least one of thetemperature and the humidity of the location where the image formingapparatus is positioned,

wherein the content of conversion of the gamma conversion means ismodulated according to the development contrast, which is the differencebetween the latent image potential causing the adhesion of the tonerflying from the development means to the electophotographicphotosensitive member and the development potential, depending on thechange amount of the gamma conversion means corresponding to the changeamount of the development amount in the result of detection by theenvironment detection means, and the weight of the gamma conversionmeans corresponding to the change amount of the contrast, in othercases.

According to the present invention, there is also provided an imageforming apparatus provided with luminance-density conversion means forconverting an original luminance signal into an original density signal,output density conversion means for converting the original densitysignal obtained by the luminance-density conversion means into an outputdensity signal, gamma conversion means for executing gamma conversion onthe output density signal obtained in the output density conversionmeans into an output signal, charging means for providing anelectrophotographic photosensitive member with a uniform charge,exposure means for exposing the electrophotographic photosensitivemember after the charging by the charging means to an exposure by theoutput signal, thereby forming a latent image, and development means fordeveloping the latent image formed by the exposure means for developingthe latent image formed by the exposure means with toner to obtain avisible image, wherein the image is rendered visible by forming a latentimage potential, which is the surface potential of theelectrophotographic photosensitive member formed by the charging meansand the exposure means and a development potential applied to thedevelopment means,

wherein the content of conversion of the gamma conversion means ismodulated according to the development contrast, which is the differencebetween the latent image potential causing the adhesion of the tonerflying from the development means to the electrophotographicphotosensitive member and the development potential.

Other objects of the present invention, and the features thereof, willbecome fully apparent from the following detailed description, which isto be taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing the configuration of the first embodiment;

FIG. 2 is a view showing the electrical configuration of a control boardand peripheral components thereof;

FIG. 3 is a view showing the configuration of an image processing unit;

FIG. 4 is a chart showing a conversion table of a luminance-densityconversion unit;

FIG. 5 is a chart showing a conversion table of an output densityconversion unit;

FIG. 6 is a chart showing a conversion table of a gamma conversion unit;

FIG. 7 is a chart showing a conversion table indicating densitycharacteristics for an output signal;

FIG. 8 is a view showing a latent image potential and a developmentpotential in a background exposure system;

FIG. 9 is a chart showing adjustment items for determining thedevelopment contrast;

FIG. 10 is a chart showing a gamma conversion table prepared in advance;

FIG. 11 is a view showing the configuration of a gamma conversion unit;

FIG. 12 is a chart showing an example of γNowi;

FIG. 13 is a chart showing a latent image potential and a developmentpotential in an image exposure system in the second embodiment;

FIG. 14 is a view showing the configuration of an image processing unitin the third embodiment;

FIG. 15 is a view showing a gamma conversion table for a dither screenprocess;

FIG. 16 is a view showing a gamma conversion table for pulse widthmodulation (PWM) processing;

FIG. 17 is a chart showing adjustment items for determining thedevelopment contrast;

FIG. 18 is a chart showing a change in the toner adhesion amount in asolid image area and in the halftone density;

FIG. 19 is a chart showing a change in the halftone density when thetoner adhesion amount in the solid image area is made constant by achange in the development contrast under an environmental fluctuation;

FIG. 20 is a chart showing two gamma conversion tables for an errordiffusion process for two development contrasts, provided in advance, inan image forming apparatus;

FIG. 21 is a view showing the configuration of a gamma conversion unitin an image processing unit in the fourth embodiment;

FIG. 22 is a chart showing an example of γNowi obtained from twodevelopment contrasts and a current development contrast;

FIG. 23 is a chart showing the latent image potential and thedevelopment potential in the image exposure system in the fifthembodiment;

FIG. 24 is a view schematically showing the configuration of an imageprocessing portion in an image forming apparatus with plural imageprocessing methods in the sixth embodiment;

FIG. 25 is a chart showing two gamma conversion tables for dither screenprocessing for two development contrasts, provided in advance, in theimage forming apparatus of the sixth embodiment; and

FIG. 26 is a chart showing two gamma conversion tables for pulse widthmodulation (PWM) processing for two development contrasts, provided inadvance, in the image forming apparatus of the sixth embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, the present invention will be clarified in detail bythe embodiments thereof, taking a laser printer as an example. However,the present invention may be realized not only by a laser printer, butalso, by any image forming apparatus of a digital type.

Also, the present invention may be realized, not only in the form of anapparatus, but also in the form of an image forming method based on thedescription of the embodiments.

First Embodiment

FIG. 1 schematically shows a laser printer constituting the firstembodiment. The printer is provided, at the approximate center of a mainbody 1, with a cylindrical photosensitive drum 2 constituting anelectrophotographic photosensitive member. The photosensitive drum 2 issupported in the main body 1 and is rotated in a direction R1, and,along the periphery of the photosensitive drum 2 and in an order of therotating direction thereof, a charge eliminating unit 3 for eliminatingthe potential on the photosensitive drum 2, a primary charger 4constituting charging means for uniformly charging the surface of thephotosensitive drum 2, exposure means 5 for exposing the surface of thephotosensitive drum 2, thereby forming an electrostatic latent image, apotential sensor 6 for measuring the potential on the photosensitivedrum 2 after exposure, a development unit 7 constituting developmentmeans for adhering toner to the electrostatic latent image, therebyforming a toner image, a transfer charger 8 for transferring the tonerimage onto a transfer material P, a separating charger 9 for separatingthe transfer material P from the photosensitive drum 2, and a cleaner 10for removing the toner remaining on the photosensitive drum 2.

The transfer material P, to which the toner image is to be transferred,is fed from a paper feeding deck 11. Below the photosensitive drum 2,namely, in the lower part of the interior of the main body 1 of theapparatus, there is provided the paper feeding deck 11 containing thetransfer material P. The transfer material P in the deck 11 is fed by afeeding roller 12, and is supplied through conveying rollers 13 andregistration rollers 15 to a space between the photosensitive drum 1 andthe transfer charger 8. In this position, the transfer material Preceives transfer of the toner image from the photosensitive drum 2, andis conveyed by a conveyor belt 16 to a fixing unit 17. After thefixation of the toner image by heat and pressure provided by the fixingunit 17, the transfer material P is discharged as the final output imageby discharge rollers 19 onto a discharge tray 20.

In the exposure means 5 of the aforementioned printer, a laser beamemitted from a semiconductor laser 30 according to an image signal isput into a scanning motion by a polygon mirror 31, and is guided throughan imaging lens 32 and a mirror 33 to the photosensitive drum 2.

FIG. 2 is a view schematically showing the electrical configuration of acontrol board for measuring the potential on the photosensitive drum 2and peripheral components. Referring to FIG. 2, a ROM storing a controlprogram and a RAM constituting a temporary memory element for the datarequired in the execution of the control program are connected to a CPU,which is the core of the process execution. Also, an interface (I/O),and A/D and D/A converters for data conversion, are connected withexternal peripheral devices, whereby the information is inputted into oroutputted from the control board. As a peripheral device, a potentialsensor is provided for measuring the potential on the photosensitivedrum after charging and after exposure. The voltage applied to theprimary charger and the output value applied to the semiconductor lasercan be controlled in order to form a desired dark voltage and lightvoltage on the photosensitive drum 2. Also, a development potential canbe formed on the development unit 7, thereby causing the toner to fly tothe photosensitive drum 2. The potential applied to the development unit7 is, in practice, composed of superimposed DC and AC components, but,in the present embodiment, the development potential is represented by aDC component.

FIG. 3 is a schematic view showing the configuration of a digital imageprocessing unit. An original image is entered into an image processingunit as an 8-bit signal, either through a reader scanner networkconstituting a reading apparatus or from a memory apparatus, such as ahard disk. The original image is subjected to a filter process in afilter process unit, in order to enhance the edge portion of the image.Then, a luminance image constituting the original image is subjected toa luminance-density conversion in a luminance-density conversion unitfor conversion into a density signal. The density signal constitutingthe original image is subjected to an output density conversion processin an output density conversion unit, for conversion into the outputdensity corresponding to an output density pattern selected by the user.The output density pattern is selected by the user on an operationpanel, not shown, in FIG. 1, but consisting a user interface, and can bea character mode in which emphasis is given to the contrast of theoutput image for outputting principally a character original, or aphotograph mode in which emphasis is given to the gradation of theoutput image for outputting principally an original photograph, or acharacter-photograph mode for outputting an intermediate image betweenthe character mode and the photograph mode. A gamma conversion unitexecutes a gamma conversion in consideration of the outputcharacteristics of the printer, on the thus obtained output densitysignal. An output signal for obtaining the output density is determinedby such gamma conversion. A table in the gamma conversion unit isconstituted by determining an inverse function for the output signal tobe outputted to the exposure system of a predetermined printer and for acorrelation table of the density signal. Such an output signal isconverted into a binary level in an error diffusion process unit forconversion from an 8-bit signal to a binary signal. Such a binary outputsignal is supplied to a semiconductor laser 30, constituting an exposuresystem, whereby a latent image, converted from the original image, isrecorded on the photosensitive drum 2. The error diffusion process unitprovides such a binary image that is microscopically a binary image, butcan macroscopically be observed as a multi-value image.

FIG. 4 shows the characteristics of a conversion table in theluminance-density conversion unit, which, being an application oflogarithmic conversion, serves to compress the signal change in a darkarea, which is not clear to the human eyes, and to expand the signalchange in a light area where the change is more easily observed by thehuman eyes.

FIG. 5 shows the characteristics of a conversion table in the outputdensity conversion unit. In the present embodiment, three tables areprovided, in which, in the character mode, giving emphasis to thecontrast of the output image in order to principally output a characteroriginal, the lighter area is more or less skipped, while, in thephotograph mode, giving emphasis to the gradation of the output image inorder to principally output an original photograph, the density signalof the original and the output density signal are in a relatively smoothlinear relationship, thereby maintaining the gradation. On the otherhand, the character-photograph mode has an interim character between thecharacter mode and the photograph mode.

FIG. 6 shows the characteristics of a conversion table in the gammaconversion unit. In the gamma conversion unit, the relationship of theoutput signal for obtaining the desired density is retained, and thegamma conversion table is given by investigating in advance the densitycharacteristics for an arbitrary output signal and determining aninverse function of a table F obtained by approximating or interpolatingsuch density characteristics with curved or straight lines, as shown inFIG. 7.

FIG. 8 is a chart showing a latent image potential and a developmentpotential in the present embodiment. Since the present embodimentemploys a background exposure method in which the toner fliesprincipally to an area not exposed to the light of the semiconductorlaser, the toner adheres to a dark potential area of the photosensitivedrum 2. A light potential area constitutes a so-called white area,though a certain fog is generated. A development potential indicates aDC component of a bias voltage applied to the development unit.Depending upon the magnitude of the development contrast, which is thedifference between the dark potential and the light potential, there arevaried the adhesion amount of the toner and the halftone density even inthe digital apparatus. In the present embodiment, there are adoptedbasic conditions of a dark potential of 300V, a light potential of 50V,a development potential of 200V and a development contrast of 190V. Inthe present embodiment, the development contrast is adjusted by theadjustment of the dark potential by a change in the applied voltage tothe primary charger. The back contrast, which is the difference betweenthe development potential and the light potential, is not changed in thepresent embodiment.

FIG. 9 is a view showing adjustment items for determining thedevelopment contrast in the present invention. The basic developmentcontrast (Vcont) is 190V, as explained in the foregoing with referenceto FIG. 8. For correcting the basic development contrast in the presentembodiment, there are added or subtracted a first development contrastcorrection amount ΔVcont1 and a second development contrast correctionamount ΔVcont2. Therefore, the total amount of the development contrastat the actual output operation by the printer is represented by:total development contrast=Vcont+ΔVcont1+ΔVcont2  (3)

The first development contrast correction amount ΔVcont1 is furtherdivided into the following two applications: namely, for reducing thetoner consumption amount and for avoiding leakage in the charger. Forthe purpose of reducing the toner consumption amount, the ΔVcont1 can beswitched to 0V (no change), −20V, −40V or −60V. A reduction of thedevelopment contrast provides the ability to reduce the adhesion amountof the toner to the dark potential, thereby lowering the tonerconsumption amount. To avoid leakage of the charger, it can be switchedto 0V (no change), −30V or −60V. The prevention of leakage means thefollowing. When the voltage applied to the primary charger is elevated,the primary charger in the printer located in a low air pressuresituation generates spark discharge, and the dark potential has to belowered in order to avoid the drawbacks resulting from such sparkdischarge.

The second development contrast correction value ΔVcont2 is also dividedinto two applications, namely, for image adjustment and forenvironmental adjustment. For the purpose of image adjustment, it can befinely set within a range from −50V to +50V. It is used by the serviceperson for the printer or the user of the printer for the purpose ofadjusting the entire image density to the darker or lighter side underthe observation of the outputted image. For the purpose of environmentaladjustment, the result of detection is read from a temperature-humiditysensor provided inside the printer for calculating the moisture in theenvironment where the printer is installed, and the development contrastis automatically and continuously adjusted from 0V to −75V, according tothe moisture. This adjustment is executed in order to avoid a phenomenonthat the development unit in the printer shows a higher developmentability in a lower moisture environment than in a higher moistureenvironment, whereby the toner adhesion amount becomes higher, even ifthe surface potential of the photosensitive drum is maintained constant.Naturally, such a tendency is affected by the developmentcharacteristics of the development unit, and, in the present embodiment,the same reduction of ΔVcont2 had to be made larger as the moisturebecomes lower.

It is to be noted that the second development contrast correction amountΔVcont2 is for image adjustment upon observation of the output image orfor environment adjustment corresponding to the state of the developmentunit, and is, therefore, to be adjusted manually or automaticallyaccording to the image. Consequently, in contrast to the firstdevelopment contrast correction value ΔVcont1, the ΔVcont2 value variesthe development contrast so as to optimize the image quality. Stateddifferently, the second development contrast correction value ΔVcont2 isused in order to decrease Dmax and the halftone density when they areexcessively high, or to increase these when they are excessively low,and the density adjustment beyond these purposes should not be executedin a digital manner.

On the other hand, the first development contrast correction amountΔVcont1 is used for altering the development contrast regardless of theimage quality. In case there is employed only one gamma conversiontable, as shown in FIG. 6, the lowering of Dmax indicating the densityof the toner adhering to the dark potential also results in a loweringof the macroscopic halftone density, thus giving a strong impression ofimage quality deterioration to the user.

Therefore, the present embodiment provides the ability to prevent theimage deterioration without lowering of the halftone density, bypreparing a gamma conversion table according to the first developmentcontrast correction amount ΔVcont1 as the digital density adjustment.

FIG. 10 shows two gamma conversion tables prepared in advance in theprinter of the present embodiment. The two gamma conversion tablesconsist of a gamma conversion table (γα1) for a case in which the firstdevelopment contrast correction amount ΔVcont1 is 0V, namely, for asituation where Vcont+ΔVcont1 is 190V (Vcontα), including the basicdevelopment contrast of 190V, regardless of the second developmentcontrast correction amount ΔVcont2, and a gamma conversion table (γβ1)for a case in which the first development contrast correction amountΔVcont1 is −50V, namely, for a situation where Vcont1+ΔVcont1 is 140V(Vcontβ), including the basic development contrast of 190V, regardlessof the second development contrast correction amount ΔVcont2. The suffixi of the gamma conversion table indicates the eight-bit value of theoutput density signal of the table and assumes a value within a range ofzero to two hundred fifty-five.

The present embodiment functions with the aforementioned two gammaconversion tables, namely, replacing the gamma conversion unit shown inFIG. 3 with that shown in FIG. 11. The gamma conversion unit shown inFIG. 11 is provided with two gamma conversion tables. Also, as shown inFIG. 9, VcontNow is determined as the sum of the basic developmentcontrast Vcont and the first development contrast correction valueΔVcont1. The VcontNow varies within a range of 190V to 70V, based on theset values shown in FIG. 9. As the reduction of the toner consumptionamount and the prevention of leakage are not used often in combination,in practice, it is generally used in the range of 190V to 130V. Aninterpolation calculation unit shown in FIG. 11 executes calculationsbased on the VcontNow, the relationship between Vcontα and Vcontβ, theconversion table at Vcontα and the conversion table at Vcontβ, accordingto an interpolation equation:γNowi=γαi+(γβi−γαi)×(Vcontα−VcontNow)/(Vcontα−Vcontβ)  (3)wherein the suffix i of the gamma conversion table is an eight-bit valueof the output density signal of the table, assuming a value within arange of zero to two hundred fifty-five. The foregoing equationnaturally assumes a value γNowi=γαi when VcontNow=Vcontα, or γNowi=γβiwhen VcontNow=Vcontβ.

FIG. 12 shows an example of the result of calculation by equation (3).In FIG. 12, γNowi is obtained by interpolation from the two γαi, γβi,the VcontNow, which is the sum of the basic development contrast Vcont,and the first development contrast correction value ΔVcont1.

The γNowi, determined by interpolation, is passed by an averagingprocess provided in the gamma conversion unit shown in FIG. 11, in orderto reduce the quantization error resulting from the deficiency in thedynamic range of the eight-bit signal.

The present embodiment employs the calculation according to equation(3), but there may also be employed an interpolating equation (4) with asuffix j:ΓNowj=Γαj+(Γβj−Γαj)×(Vcontα−VcontNow)/(Vcontα−Vcontβ)  (4)wherein Γ indicates a table of the output density signal correspondingto the output signal j and is an inverse function to γ.

The above-described embodiment employing the gamma conversion tableaccording to the development contrast has the features shown in thefollowing Table 1, in which a comparison is made with a conventionalexample 1, in which the analog image adjustment alone is executed, forexample, by the change of the development contrast only, and with aconventional example 2, in which the digital image adjustment alone isexecuted, for example, by an arbitrary change of the gamma conversiontable only. The analog image adjustment, as in the conventional example1, results in a lowering of Dmax, and at the same time, in a lowering ofthe halftone density, thereby significantly deteriorating the apparentimage quality. The lowered density itself results in a phenomena such asa weaker impression of the image, deteriorated gradation and thinning ofthe lines. On the other hand, the digital image adjustment, as in theconventional example 2, shows a weaker effect of toner reduction, incomparison with the analog image adjustment, thereby resulting in anextremely large loss of the halftone density, in order to achieve thedesired toner reducing effect, and has no effect in preventing theleakage of the charger.

In contrast, in the present embodiment, the gamma conversion table isautomatically modulated (change of the gamma conversion characteristics)simultaneously with the lowering of the development contrast, therebymaintaining the halftone density, though Dmax is lowered, thus avoidingpoor impression of the image. This is due to a fact that the poorimpression is not caused because of the weaker response of the humaneyes to the density in the darker area, as long as Dmax in thereflective density is at least equal to 1.1. Also, the presentembodiment providing the ability to achieve reduction of the toneramount and prevention of the leakage at the same time, thereby providingthe ability to attain the image quality and other requirements at thesame time, unlike the conventional examples 1 and 2 (see Table 1 below).

TABLE 1 Conventional Conventional Present example 1 example 2 EmbodimentAnalog used — used (development contrast lowered) Digital (gamma — usedAutomatically conversion table modulated modulated) according todevelopment contrast Dmax lowered maintained lowered Halftone densitylow significantly maintained lower Image in total apparently poor uponlittle influenced poor toner amount by lower Dmax reduction Drawbackdefects in leakage cannot none image be prevented

As explained in the foregoing, the present embodiment provides theability to maintain the image quality as long as possible, even in asituation where a lowering of the development contrast is unavoidable,such as in reducing the toner consumption amount or avoiding the leakageof the charger, by preparing gamma conversion tables corresponding totwo development contrasts, and calculating a suitable gamma conversiontable according to the current development contrast.

Second Embodiment

The foregoing first embodiment has been explained by a printer of thebackground exposure type, in which the toner principally flies to thedark potential area, but the present invention is likewise applicable toa printer of the image exposure type, in which the toner principallyflies to the light potential area, as shown in FIG. 13, and such anexample will be explained as the second embodiment.

In the image exposure system, the difference of the developmentpotential and the light potential constitutes the development contrast,and the difference between the dark potential and the developmentpotential constitutes the back potential. In the present embodiment,there are assumed basic conditions of a dark potential of 400V, a lightpotential of 50V, a basic development potential of 250V, and adevelopment contrast of 200V.

In the first embodiment, the development contrast is modulated by themodulation of the dark potential, but, in the present embodiment, thedevelopment contrast is modulated simultaneously with the darkpotential. This is because the dark potential is not directly related tothe development contrast in the image exposure system, in contrast tothe first embodiment, in which the dark potential has to be lowered forpreventing the leakage, as shown in FIG. 9. Therefore, the back contrastremains unchanged by varying the development potential corresponding tothe lowering of the dark potential, whereby the development contrastalone can be lowered without a variation, such as an increase or adecrease of the fog level. The back contrast relating to the fog level,if made smaller, results in unnecessary adhesion of the toner calledbackground fog, but, if made excessively large, results in a reversalfog induced by the toner, which is charged in an opposite polarity or animage stain called shadowing. It is, therefore, important to maintainthe back contrast within a certain range, and the back contrast can beadvantageously maintained constant by reducing the development potentialin linkage with the dark potential. The printer of the presentembodiment utilizing the lowered development potential provides theability, as in the first embodiment, to prevent the leakage, to reducethe toner consumption amount and to provide the high image quality, atthe same time.

In the first embodiment, the present invention is realized by a printerof the background exposure type, but, the present invention, consideredfrom the standpoint of the development contrast, can also be realized,as in the present embodiment, in the printer of an image exposuresystem. More specifically, by reducing the development potentialsimultaneously with the lowering of the dark potential, it is possibleto prevent leakage of the charger and reduction of the toner consumptionamount resulting from the lowered development contrast, and also, tosuppress the loss in the image quality by the optimum gamma conversion.

Third Embodiment

In the first embodiment, shown in FIG. 3, the conversion to the binarylevel is executed in the error diffusion unit after the gammaconversion. In the present embodiment, there will be explained a printerutilizing plural image processing methods, including the error diffusionprocess. FIG. 14 is a schematic view showing the configuration of adigital image processing unit of the present embodiment, in which anoriginal luminance signal read from a reader, a scanner, a network or amemory device, such as a hard disk, is subjected to an edge enhancementin a filter unit, then converted into an original density signal in aluminance-density conversion unit, and further converted into a desiredoutput density signal in an output density conversion unit. The presentembodiment is provided, for preparing signal data for driving thesemiconductor laser, with an error diffusion process explained in thefirst embodiment, a dither screen process, and a pulse width modulation(PWM) process.

The present embodiment is featured by preparing two gamma conversiontables for each of the aforementioned three image processing methods, orsix gamma conversion tables in total, and preparing a gamma conversiontable corresponding to the current development contrast from the twogamma conversion tables corresponding to the image processing method. Inthe dither screen process, the error diffusion process tends to generatemore scattered toner dots on the photosensitive drum, thereby producingfiner dots. For this reason, the gamma conversion table becomesdifferent, even for a same development contrast. Similarly, the PWMprocess, having a known tendency of generating dots more scattered inthe main scanning direction and more connected in the sub scanningdirection, requires gamma conversion tables different from those for theerror diffusion process or for the dither screen process. Consequently,gamma conversion tables are necessary, corresponding to each of thethree image processing methods. The error diffusion process is used, forexample, in the character mode, because of its features that thestability of gradation is inferior to that in other processes, thoughthe resolution is extremely high. On the other hand, the dither screenprocess is used, for example, in the photograph mode, because of theexcellent stability of the gradation, though the resolution is inferior.The PWM process is positioned in properties between these two methodsand is, therefore, used, for example, in the character photograph mode.

FIG. 15 shows gamma conversion tables for two development contrasts foruse in the dither screen process, and FIG. 16 shows gamma conversiontables for two development contrasts for use in the PWM process. Thegamma conversion unit is provided in advance with two gamma conversiontables for each of the three image processing methods, including thoseshown in FIG. 10, and a gamma conversion table corresponding to thecurrent development contrast is prepared through an interpolation unitand an averaging unit. The present embodiment, in which an optimum gammaconversion table is automatically selected corresponding to the imageprocessing method and the current development contrast, is capable ofproviding constantly stable images, even in a situation where thedevelopment contrast has to be lowered.

As explained in the foregoing, the present embodiment provides, in aprinter provided with plural image processing methods, an optimum imageaccording to the image processing method and the current developmentcontrast, by preparing two gamma conversion tables for each of the imageprocessing methods.

The foregoing embodiment provides an image with little deterioration,even when an adjustment of the image contrast is unavoidable, not forthe purpose of image adjustment.

Fourth Embodiment

In the foregoing first to third embodiments, the gamma (γ) developmentcontrast VcontGamma (described as VcontNow in the first to thirdembodiments) for gamma conversion is practically defined as follows:VcontGamma=Vcont+A·Δcont1+B·ΔVcont2wherein A=1, B=0, thereforeVcontGamma=Vcont+ΔVcont1  (5)

Therefore, it is defined solely by the first development contrastcorrection amount ΔVcont1 for correcting the development contrast andthe basic development contract Vcont. The first development contrastcorrection amount is determined by the reduction of the tonerconsumption amount in the development means or by the prevention of theleakage in the charging means, and the deterioration of the image can besuppressed, even when the development contrast has to be adjusted, bychanging the content of conversion of the gamma conversion meansaccording to equation (5). On the other hand, the second developmentcontrast correction amount is determined by the adjustment of the imagedensity or the adjustment for the environmental fluctuation, and is notmade to contribute to the content of conversion of the gamma conversionmeans.

However, there may be encountered a following drawback if the reductionof the toner consumption amount or the prevention of the leakage of thecharging means alone is correlated with the content of conversion of thegamma conversion means, as indicated by equation (5).

An environmental fluctuation in the development characteristics causes achange not only in the halftone density, but, also, in the toneradhesion amount, even under a same development contrast. A change in thehalftone density may be recognized as a deterioration in the imagequality. Also, a change in the toner adhesion amount may affect thetoner consumption amount and may induce a drawback, in a case ofexcessive adhesion, for example, scattering of the image or imagedeterioration in a portion where the toner surface is subjected tofriction, such as an enhanced trace of a separating finger of the fixingroller. In connection with such a phenomenon, the experiments of thepresent inventors have clarified that the changes in the halftonedensity and in the toner adhesion amount, resulting from theenvironmental fluctuation do not take place at a same level. In theimage forming apparatus employing digital area gradation, the halftonedensity is generally determined by the toner deposition rate per area ofthe sheet, while the toner adhesion amount determining the imagedeterioration caused by the toner scattering or the trace of theseparating finger can be regarded as the toner amount in the heightdirection with respect to the sheet. It has been found that the tonerdeposition under the environmental fluctuation does not change with aconstant rate in the area and height directions of the sheet, butchanges individually in these directions. Therefore, if the totaldevelopment contrast VcontBody of the main body is determined so as tooptimize the toner adhesion amount for the environment fluctuation and,if the content of the conversion of the gamma conversion means isdetermined without taking the environmental fluctuation intoconsideration, the halftone density cannot reach the target density, sothat the desired toner adhesion amount and the desired halftone densitycannot be attained at the same time. If the priority is given to thehalftone density, naturally, the total development contrast VcontBody ofthe main body does not become appropriate, so that the inadequate toneradhesion amount results in an unsatisfactory toner consumption amount orimage deterioration, such as toner scattering or a trace of theseparating finger.

In the following embodiment, therefore, there are conceived thefollowing means for attaining the toner adhesion amount and the halftonedensity at the same time. More specifically, the development contrast inthe main body is adjusted so as to optimize the toner adhesion amount,in consideration of the influence of the environmental fluctuation. Atthe same time, as the halftone density also shows a change, though notin direct correlation, the development contrast based on theenvironmental fluctuation is corrected with a weight. More specifically,the gamma development contrast VcontGamma for gamma conversion isdefined in the following manner:VcontGamma=Vcont+A·ΔVcont1+B·ΔVcont2+C·ΔVcont3wherein A=1, B=0, and C is a non-zero constant, therefore:VcontGamma=Vcont+ΔVcont1+C·ΔVcont3  (6)

The first development contrast correction amount ΔVcont1 is used forreducing the toner consumption amount in the development means or forpreventing the leakage in the charging means, while the seconddevelopment contrast correction amount ΔVcont2 is used for adjusting theimage density, and the third development contrast correction amountΔVcont3 is used for an adjustment for the environmental fluctuation.Since the constant C is larger than zero, but smaller than one, thechange in the halftone density resulting from the environmentfluctuation can be corrected in an optimum manner.

FIG. 17 shows the adjustment items for determining the developmentcontrast. The basic development contrast (Vcont) is 190V, as explainedin FIG. 8. In the present embodiment, such a basic development contrastis corrected by adding or subtracting three correction amounts, namely,the first development contrast correction amount ΔVcont1, the seconddevelopment correction amount ΔVcont2 and the third development contrastcorrection amount ΔVcont3 to or from the basic development contrast.Consequently, the total development contrast VcontBody at the actualoutput operation of the image forming apparatus is represented by:VcontBody=Vcont+ΔVcont1+ΔVcont2+ΔVcont3  (7)

Also, in the present embodiment, the gamma development contrast forgamma conversion is defined according to equation (6).

Also, the development contrast, not including the second developmentcontrast correction amount ΔVcont2, is defined as VcontGamma accordingto equation (6), as will be explained later.

The first development contrast correction amount ΔVcont1 is furtherdivided into the following two applications: namely, for reducing thetoner consumption amount and for avoiding leakage in the charger. Forthe purpose of reducing the toner consumption amount, the value ofΔVcont1 can be switched to 0V (no change), −20V, −40V or −60V. Areduction of the development contrast reduces the adhesion amount of thetoner to the dark potential, thereby lowering the toner consumptionamount. For the purpose of avoiding leakage of the charger, it can beswitched to 0V (no change), −30V or −60V. The leakage prevention meansthe following. When the voltage applied to the primary charger iselevated, the primary charger in the printer located in a low airpressure situation generates a spark discharge, and the dark potentialhas to be lowered in order to avoid the drawbacks resulting from such aspark discharge.

The second development contrast correction value ΔVcont2 is used forimage adjustment, and can be finely set within a range from −50V to+50V. It is used by the service person for the image forming apparatusor the user thereof for the purpose of adjusting the entire imagedensity to the darker or lighter side under the observation of theoutputted image.

The third development contrast correction value ΔVcont3 is used forenvironmental adjustment. For the environmental adjustment, the resultof the detection is red from a temperature-humidity sensor providedinside the image forming apparatus for calculating the moisture in theenvironment where the printer is installed, and the development contrastis automatically and continuously adjusted from 0V to −50V, according tothe moisture. This adjustment is executed in order to avoid a phenomenonthat the development unit in the image forming apparatus shows a higherdevelopment ability in a lower moisture environment than in a highermoisture environment, whereby the toner adhesion amount becomes higher,even if the surface potential of the photosensitive drum is maintainedconstant. Naturally, such a tendency is affected by the developmentcharacteristics of the development unit, and, in the present embodiment,the amount of reduction of ΔVcont3 had to be made smaller as themoisture level becomes lower.

The first development contrast correction amount ΔVcont1 is to changethe development regardless of the image quality. In case only one gammaconversion table, shown in FIG. 6, is employed, there will result alowering of the macroscopic halftone image, as well as a lowering inDmax indicating the density of the toner adhering to the dark potential,thus inducing an impression of significant image deterioration to theuser. For this reason, equation (7) determines the development contrastof the image forming apparatus and equation (6) determines the gammadevelopment contrast for gamma conversion, including ΔVcont1 with a samecontribution.

The second development contrast correction amount ΔVcont2 is used forimage adjustment upon confirmation of the output image and is adjustedmanually according to the image. Therefore, a change in the developmentcontrast serves to optimize the image quality. Stated differently, thesecond development contrast correction value ΔVcont2 is used in order todecrease Dmax and the halftone density when they are excessively high,or to increase these when they are excessively low, and the densityadjustment beyond these purposes should not be executed in a digitalmanner. Therefore, though equation (7) includes ΔVcont2, it isunnecessary in equation (6) for determining the gamma developmentcontrast gamma conversion.

FIG. 18 shows the influence of the environmental fluctuation on thedevelopment characteristics, by the toner adhesion amount in a solidimage area and by the density in a halftone area. It shows the variationrates of the solid area toner adhesion amount and the halftone density,taking the values thereof as 100 at a high moisture (22 g/kg), to alower moisture situation (5 g/kg). For a lower moisture, the developmentunit shows a higher development ability, with increases both in thesolid area toner adhesion amount and the halftone density, but thechange rate is not the same for the two. FIG. 18 shows that the changerate is higher for the solid area toner adhesion amount than for thehalftone density.

Therefore, if the development contrast of the main body of the imageforming apparatus is changed, according to the moisture, so as tostabilize the toner adhesion amount in the solid image area, thehalftone density does not become constant, as shown in FIG. 19. It willalso be understood that, if the development contrast of the main body ofthe image forming apparatus is changed, according to the moisture, so asto stabilize the halftone density, the toner adhesion amount in thesolid image area does not become constant. Thus, if priority is given tothe halftone density as in the conventional examples, there cannot beavoided the fluctuation in the toner adhesion amount, eventually leadingto an increased toner consumption amount, a scattering of the image or atrace of the separating finger remaining on the sheet.

According to an experiment of the present inventors, the ratio of thechange rates of the solid area toner adhesion amount and the halftonedensity, as shown in FIG. 18, is 7:2 in the image forming apparatusemployed for the experiment. This value may naturally be different inanother image forming apparatus of another image forming system. Theabove-mentioned value means that, in a case of determining the potentialconditions for providing the optimum image characteristics under a highhumidity environment, for example, of 22 g/kg and for obtaining the sameimage characteristics under a low humidity environment, for example, of5 g/kg, the development contrast has to be reduced by 50V for the solidarea toner adhesion amount and by about 15V for the halftone density.Stated differently, if the development contrast is reduced by 50V, inorder to stabilize the solid area toner adhesion amount in a case of ashift from a high humidity environment to a low humidity environment,the halftone density is recovered partially corresponding to 15V, but isstill deficient corresponding to the remaining 35V (=50V−15V).

In the present invention, therefore, the half tone density is assured bya feedback of a portion corresponding to such 35V, not to the contrastpotential, but to the content of conversion of the digital gammaconversion means. This is represented by a term C·ΔVcont3 in equation(2). In the present embodiment, an optimum result can be obtained in acase C=0.714 (=35/50V=5/7V), so that equation (6) becomes:VcontGama=Vcont+ΔVcont1+0.714·ΔVcont3  (6′)

In the present invention, it is possible to suppress the imagedeterioration without losing the halftone density, by preparing gammaconversion tables according to VcontGamma in equation (6′).

FIG. 20 shows two gamma conversion tables prepared in advance in theimage forming apparatus of the present invention and consisting of agamma conversion table (yai) for a case where the contrast correctionamounts ΔVcont1, ΔVcont2 and ΔVcont3 are all 0V, namely, a case ofVcontGamma=Vcont=190V (Vcontα), and a gamma conversion table (γβi) for acase where Vcont is 140V (Vcontβ), and a suffix i of the gammaconversion table indicates an eight-bit value of the output densitysignal of the table, assuming a value within a range of zero to twohundred fifty-five.

The present embodiment functions with the aforementioned two gammaconversion tables, namely, replacing the gamma conversion unit shown inFIG. 3 with that shown in FIG. 21. The gamma conversion unit shown inFIG. 21 is provided with two gamma conversion tables. Also, as shown inthe equation (6′), VcontGamma is determined as the sum of the basicdevelopment contract Vcont and 0.714 times of the third developmentcontrast correction value ΔVcont3. The VcontGamma can vary by about 155Vfrom the set value shown in FIG. 17, namely, within a range of 190V to35V. As the reduction of the toner consumption amount and the preventionof leakage are not used often in combination, in practice, it isgenerally used in the range of 190V to 100V. An interpolationcalculation unit shown in FIG. 21 executes calculation based on theVcontGamma, the relationship between Vcontα and Vcontβ, the conversiontable at Vcontα and the conversion table at Vcontβ, according to theinterpolation equation:γNowi=γαi+(γβi−γαi)×(Vcontα−VcontGamma)/(Vcontα−Vcontβ)  (8)wherein the suffix i of the gamma conversion table is an eight-bit valueof the output density signal of the table, assuming a value within arange of zero to two hundred fifty-five. The foregoing equationnaturally assumes a value γNowi=γαi in case VcontGamma=Vcontα, orγNowi=γβi in case VcontGamma=Vcontβ.

FIG. 21 shows an example of the result of calculation by equation (8).In FIG. 12, γNowi is obtained by interpolation from the two gammaconversion tables γαi, γβi, and the VcontGamma, which is the current sumof the basic development contrast Vcont and the first developmentcontrast correction value ΔVcont1.

The γNowi, determined by interpolation, is passed by an averagingprocess provided in the gamma conversion unit shown in FIG. 21 in orderto reduce the quantization error resulting from the deficiency in thedynamic range of the eight-bit signal.

The present embodiment employs the calculation according to equation(8), but there may also be employed an interpolating equation (9) with asuffix j:ΓNowj=Γαj+(Γβj−Γαj)×(Vcontα−VcontGamma)/(Vcontα−Vcontβ)  (9)wherein Γ indicates a table of the output density signal correspondingto the output signal j and is an inverse function to γ.

In the exploitation of the present invention, the gamma conversiontables prepared in advance and the current ones derived from equation(8) or equation (9) are subjected to sufficient smoothing at thepreparation of the tables prepared in advance, or at the derivation ofthe current tables, in order not to cause a jumping in the imagegradation by the quantization error.

As explained in the foregoing, the present invention is capable ofindividually optimizing the development characteristics under theinfluence of the environmental fluctuation and provides the featuresshown in the following Table 2, in comparison with the conventionalexamples. In the conventional examples, the change in the developmentcharacteristics induced by the environmental fluctuation is not takeninto consideration sufficiently, and the development contrast is merelychanged so as to stabilize the halftone density, but the toner adhesionamount becomes excessively high in the solid image area, because of thedifference in the change rate, from the halftone density, under theenvironmental fluctuation. Such excessive toner adhesion leads todrawbacks, such as an increased toner consumption amount, scattering ofthe image in the development, transferring or fixing unit, and anenhanced trace of the fixing finger in the solid image area. On theother hand, in the present invention, the development contrast ischanged according to the toner adhesion amount in the solid image area,and the gamma conversion tables are prepared so as to compensate for thedeficiency in the halftone image. Consequently, there can besimultaneously attained the toner adhesion amount in the solid imagearea and the halftone density, and there can be provided an imageforming apparatus resistant to the environmental fluctuations (see Table2 below):

TABLE 2 Conventional Present example Embodiment Consideration on Changein Change in development development development characteristics bycontrast only contrast and environmental modulation of gamma fluctuationconversion table with weighting Solid area toner Stabilized byStabilized by adhesion amount development development contrast contrastHalftone density lower Stabilized by gamma conversion table DrawbackPoor image by none lowered halftone density

In the present embodiment, equation (6′) is defined for an image formingapparatus showing higher values of the solid area toner adhesion amountand the halftone density at a lower humidity environment, but theconstant C may assume a negative value depending on the image formingsystem of the image forming apparatus. Also, there has been explained acase where the change in the solid area toner adhesion amount is largerthan that in the halftone density, but it will be apparent that thepresent invention is applicable also in an opposite case.

Also, in the present embodiment, an environment sensor is provided asthe environment detection means and the moisture in the environmentwhere the image forming apparatus is installed is measured from thetemperature and humidity from such a sensor, but a sensor for thetemperature only or for the humidity only may be employed for thepurpose of cost reduction.

Fifth Embodiment

The foregoing fourth embodiment has been explained by an image formingapparatus of the background exposure type in which, as shown in FIG. 8,the toner principally flies to the dark potential area, but the presentinvention is likewise applicable to an image forming apparatus of theimage exposure type in which the toner principally flies to the lightpotential area, as shown in FIG. 23.

In the image exposure system, the difference of the developmentpotential and the light potential constitutes the development contrast,and the difference between the dark potential and the developmentpotential constitutes the back potential. In the present embodiment,there are assumed basic conditions of a dark potential of 400V, a lightpotential of 50V, a basic development potential of 250V, and adevelopment contrast of 200V.

In the fourth embodiment, the development contrast is modulated by themodulation of the dark potential, but, in the present embodiment, thedevelopment contrast is modulated simultaneously with the darkpotential. This is because the dark potential is not directly related tothe development contrast in the image exposure system, in contrast tothe fourth embodiment, in which the dark potential has to be lowered forpreventing the leakage, as shown in FIG. 17. Therefore, the backcontrast remains unchanged by varying the development potentialcorresponding to the lowering of the dark potential, whereby thedevelopment contrast alone can be lowered with the variation, such as anincrease or a decrease of the fog level. The back contrast relating tothe fog level, if made smaller, results in unnecessary adhesion of thetoner, called background fog, but, if made exclusively large, results ina reversal of fog induced by the toner, which is charged in an oppositepolarity or in an image stain called shadowing. It is, therefore,important to maintain the back contrast within a certain range, and theback contrast can be advantageously maintained constant by reducing thedevelopment potential in linkage with the dark potential. The imageforming apparatus of the present embodiment, utilizing the lowereddevelopment potential, provides the ability, as in the fourthembodiment, to attain the prevention of the leakage, the reduction ofthe toner consumption amount and the high image quality at the sametime, and, even under an environmental fluctuation at the same time, theimage is not deteriorated, because the development contrast is modulatedfor stabilizing the solid area toner adhesion amount, and the gammaconversion table is modulated for stabilizing the halftone density.

Therefore, the present invention employing the gamma conversioncorresponding to the development contrast is applicable also to theimage forming apparatus of the image exposure type.

Sixth Embodiment

In the fourth embodiment, as shown in FIG. 3, the conversion to thebinary level is executed in the error diffusion unit after the gammaconversion. In the present embodiment, there will be explained an imageforming apparatus utilizing plural image processing methods includingthe error diffusion process. FIG. 24 is a schematic view showing theconfiguration of a digital image processing unit of the presentembodiment, in which an original luminance signal read from a reader, ascanner, a network or a memory device, such as a hard disk, is subjectedto an edge enhancement in a filter unit, then converted into an originaldensity signal in a luminance-density conversion unit, and furtherconverted into a desired output density signal in an output densityconversion unit. The present embodiment is provided, for preparingsignal data for driving the semiconductor laser, with an error diffusionprocess explained in the fourth embodiment, a dither screen process, anda pulse width modulation (PWM) process.

The present embodiment is featured by preparing two gamma conversiontables for each of the aforementioned three image processing methods, orsix gamma conversion tables in total, and preparing a gamma conversiontable corresponding to the current development contrast from two gammaconversion tables corresponding to the image processing method. In thedither screen process, the error diffusion process tends to generatemore scattered toner dots on the photosensitive drum, thereby producingfiner dots. For this reason, the gamma conversion table becomesdifferent, even for the same development contrast. Similarly, the PWMprocess, having an already known tendency of generating dots morescattered in the main scanning direction and more connected in the subscanning direction, requires gamma conversion tables different fromthose for the error diffusion process or for the dither screen process.Consequently, there are required two gamma conversion tablescorresponding to each of the three image processing methods. The errordiffusion process is used, for example, in the character mode, becauseof its features that the stability of gradation is inferior to that inother processes, though the resolution is extremely high. On the otherhand, the dither screen process is used, for example, in the photographmode, because of the excellent stability of the gradation, though theresolution is inferior. The PWM process is positioned in propertiesbetween these two methods and is, therefore, used, for example, in thecharacter photograph mode.

FIG. 25 shows gamma conversion tables for two development contrasts foruse in the dither screen process, and FIG. 26 shows gamma conversiontables for two development contrasts for use in the PWM process. Thegamma conversion unit is provided in advance with two gamma conversiontables for each of the three image processing methods, including thoseshown in FIG. 6, and a gamma conversion table corresponding to thecurrent development contrast is prepared through an interpolation unitand an averaging unit. The present embodiment, in which an optimum gammaconversion table is automatically selected corresponding to the imageprocessing method and the current development contrast, is capable ofproviding constantly stable images, even in a situation where thedevelopment contrast has to be lowered.

In the fourth to sixth embodiments, there are prepared in advance thegamma conversion tables corresponding to the two development contrasts,and an optimum gamma conversion table is calculated according to thecurrent development contrast, whereby the image quality represented bythe solid area toner adhesion amount and the halftone density can bestabilized not only in a situation where the development contrast has tobe lowered, such as in the reduction of the toner consumption amount orin the prevention of the leak of the charger, but also, under anyenvironment (as described in the first embodiment).

In the fourth embodiment, there has been explained an image formingapparatus of the background exposure system, but the present inventionis also applicable to an image forming apparatus of the image exposuresystem, taking the development contrast as a parameter. Thus, bylowering the development potential simultaneously with the lowering ofthe dark potential, the image quality represented by the solid areatoner adhesion amount and the halftone density can be stabilized, notonly in a case of reducing the toner consumption amount or in a case ofpreventing leakage by the charger, but also, under any environment (asdescribed in the fifth embodiment).

Also, in the image forming apparatus provided with plural differentimage processing methods, an optimum image can be provided according tothe image processing method and the development contrast, by preparingtwo gamma conversion tables for each of the image processing methods (asdescribed in the sixth embodiment).

The present invention has been explained by certain preferredembodiments thereof, but the present invention is not limited to suchembodiments, and is subject to various modification and applicationswithin the scope and spirit of the appended claims.

1. An image forming apparatus, which visualizes an image, according to alatent image potential, on an image bearing body, which includes acharging unit and an exposing unit, in which a development potential isapplied to a developing unit, and a development contrast is a differencebetween the latent image potential and the development potential,wherein a designated development contrast is defined as a sum of a basicdevelopment contrast Vcont, and a development correction amount ΔVcont1,for correcting the basic development contrast Vcont, said image formingapparatus comprising: (a) a γ conversion unit having a γ conversiontable γα_(i) of a first value Vcontα of the designated developmentcontrast in a case that the development contrast correction amountΔVcont1 is a first value, and a γ conversion table γβ_(i), differentfrom the γ conversion table γα_(i), of a second value Vcontβ, differentfrom the first value Vcontα, of the designated development contrast in acase that the development contrast correction amount ΔVcont1 is a secondvalue different from the first value of ΔVcont1; and (b) an arithmeticoperation unit for calculating a γ conversion table γNow_(i) of a thirdvalue VcontNow of the designated development contrast in a case that thedevelopment contrast correction amount ΔVcont1 is a third value, basedon a formula:γNow_(i)=γα_(i)+(γβ_(i)−γα_(i))×(Vcontα−VcontNow)/(Vcontα−Vcontβ),wherein the subscript i is a value of an output density signal in the γconversion table, and wherein the γ conversion unit conducts γconversion of the third value VcontNow of the designated developmentcontrast, based on the γ conversion table γNow_(i) calculated by thearithmetic operation unit.
 2. The image forming apparatus according toclaim 1, wherein the first development contrast correction amountΔVcont1 is used to reduce a toner consumption amount in the developingunit or to deal with leakage of the charging unit, and the firstdevelopment contrast correction amount may be one of a plurality oflevels.
 3. The image forming apparatus according to claim 1, wherein thesecond development contrast correction amount ΔVcont2 is used to adjustimage density.
 4. An image forming apparatus according to claim 1,further comprising: an environment detection unit adapted to detect atleast one of a temperature and a humidity; and an adjustment unitadapted to execute image adjustment by changing a target developmentcontrast and a gamma development contrast for gamma conversion, andexecuting the gamma conversion based on the changed gamma developmentcontrast, according to a detected result of the environment detectionunit, wherein the adjustment unit obtains the target developmentcontrast VcontBody by a sum total of a basic development contrast Vcont,a first development contrast correction amount ΔVcont1 for correctingthe basic development contrast, a second development contrast correctionamount ΔVcont2, and a third development contrast correction amountΔVcont3, such that VcontBody=Vcont+ΔVcont1+ΔVcont2+ΔVcont3, theadjustment unit obtains the gamma development contrast VcontGamma forgamma conversion by VcontGamma=Vcont+ΔVcont1+C·ΔVcont3, where C is aconstant other than 0, and setting of C is defined within a range 0<C<1according to the detected result of the environment direction unit. 5.The image forming apparatus according to claim 4, wherein the thirddevelopment contrast correction amount ΔVcont3 is used to adjust avariation due to environment.